A process using, as an olefin polymerization catalyst, a transition metal compound having a cyclopentadienyl ligand or a substituted cyclopentadienyl ligand, namely, a so-called metallocene compound, is widely known. Since it was reported by W. Kaminsky, et al. that a catalyst using a combination of zirconocenedimethyl and methylalumioxane (MAO) exhibits high activityinpolymerizationof ethylene [Angew. Chem. Int. Ed. Engl., 19, 390 (1980)], various improvements have been attempted for the purpose of enhancing performance of a catalyst, producing a specific polymer, etc. With regard to a process for stereoregularly polymerizing an α-olefin among them, there have been reported isotactic polymerization by W. Kaminsky, et al. [Angew. Chem. Int. Ed. Engl., 24, 507 (1985)] and syndiotactic polymerization by J. A. Ewen, et al. [J. Am. Chem. Soc., 110, 6255 (1988)] one after another in 1980s, and both of them have been accomplished by utilizing a specific stereostructure of a crosslinked metallocene compound. Particularly with regard to the latter, production of syndiotactic polypropylene that was difficult to produce with a conventional Ziegler-Matta catalyst has been succeeded by the use of a metallocene compound having a ligand in which a cyclopentadienyl group and a fluorenyl group have been crosslinked.
Thereafter, development of this crosslinked cyclopentadienyl-fluorenyl metallocene compound also as a catalyst for ethylene homopolymerization or ethylene/α-olefin copolymerization has been advanced. W. Kaminsky, et al. have reported ethylene polymerization using [isopropylidene(η5-cyclopentadienyl)(η5-fluorenyl)]zirconium dichloride that was used for the production of syndiotactic polypropylene by J. A. Ewen, et al. previously mentioned, but its polymerization activity was extremely low [Makromol. Chem., 193, 1643 (1992)].
On the other hand, the present applicant has earnestly studied ligand structures and has reached an invention of a crosslinked metallocene compound having extremely high polymerization activity in ethylene homopolymerization and ethylene/α-olefin copolymerization [patent literature 1 (WO2004/029062), patent literature 2 (WO2005/100410)]. However, the molecular weight of the resulting ethylene-based polymer is still insufficient, and further improvement in the catalyst has been desired.
In solution polymerization, it is generally regarded as preferable to carryout polymerization at a high temperature because this leads to enhancement in productivity. That is to say, since the viscosity of a polymer solution containing the resulting olefin polymer is decreased at a high temperature, it becomes possible to raise a concentration of the olefin polymer in a polymerizer as compared with polymerization at a low temperature, and as a result, productivity per polymerizer is enhanced. Further, the olefin polymerization is an exothermic reaction, and therefore, in order to maintain the polymerization temperature at a desired value, heat of polymerization usually needs to be removed. In the high-temperature polymerization, the quantity of heat to be removed is smaller than that in the low-temperature polymerization, and therefore, an advantage of reduction in cost of heat removal is also obtained. On the other hand, it is well known to a person skilled in the art that the molecular weight of the resulting olefin polymer decreases with a rise in polymerization temperature. Accordingly, a disadvantage that the upper limit of the polymerization temperature is restricted to produce an olefin polymer having a desired molecular weight occurs frequently. As a means to eliminate this disadvantage, a polymerization catalyst for producing an olefin polymer having a high molecular weight is desired. By the use of such an olefin polymerization catalyst, it becomes possible to maintain the molecular weight of the resulting olefin polymer at a desired high value in the high-temperature polymerization, and advantages of enhancement in productivity and reduction in production cost are obtained.
Catalysts for producing such an olefin polymer having a high molecular weight and improvements regarding metallocene compounds that constitute the catalysts have been studied so far. It is widely known that in various metallocene compounds of transition metals of Group 4 of the periodic table, a hafnium compound produces an olefin polymer having a higher molecular weight as compared with a zirconium compound having the same structure as the hafnium compound. In Japanese Patent Publication No. 1994-811 and the like, it is disclosed that by the use of hafnocene dichloride as a metallocene compound, a molecular weight of the resulting polyethylene is increased as compared with zirconocene dichloride. Similarly to the above, it is disclosed in Japanese Patent No. 2882257 that by the use of [isopropylidene(η5-cyclopentadienyl)(η5-fluorenyl)]hafnium dichloride, a molecular weight of the resulting ethylene/1-hexene copolymer is increased as compared with [isopropylidene(η5-cyclopentadienyl)(η5-fluorenyl)]zirconium dichloride. In either case, however, the molecular weight of the resulting olefin polymer is not sufficient, and it is difficult to produce an olefin polymer having a desired molecular weight at such a high temperature as is industrially useful.
W. Kaminsky, et al. have further made improvements, and by introducing substituents into a crosslinked part and a fluorenyl group part of a crosslinked cyclopentadienyl-fluorenyl metallocene compound, enhancement in molecular weight of the resulting polypropylene has been attempted [J. Organomet. Chem., 684, 200 (2003)]. Although a certain result has been achieved by this attempt, a tendency to decrease in molecular weight of the resulting polypropylene with a rise in polymerization temperature is marked, and in the aimed high-temperature polymerization, polypropylene having a desired molecular weight has not been obtained yet.
The present applicant has proposed a process for producing an α-olefin polymer using a catalyst comprising a specific crosslinked cyclopentadienyl-fluorenyl metallocene compound in a patent literature 3 (WO2006/123759). According to this process, when α-olefins at least partially containing ethylene are polymerized under a high-temperature condition, an ethylene-based polymer having a high molecular weight can be produced with a good activity. In the patent literature 3, further, the present applicant has proposed a process for producing a propylene-based copolymer using a catalyst comprising a different specific crosslinked cyclopentadienyl-fluorenyl metallocene compound. According to this process, a propylene-based copolymer having a high molecular weight can be efficiently produced, and a propylene-based copolymer having a desired molecular weight can be produced at a higher temperature than that in the case using a conventional olefin polymerization catalyst.
On the other hand, a method of introducing hydrogen into a polymerization reactor and thereby lowering a molecular weight of the olefin polymer in order to produce an olefin polymer having a desired molecular weight is popular to a person skilled in the art. For example, the present applicant has disclosed that by introducing hydrogen into a polymerization reactor in the copolymerization of ethylene and 1-octene using a polymerization catalyst comprising a crosslinked cyclopentadienyl-fluorenyl metallocene compound, a molecular weight of the resulting ethylene/1-octene copolymer is lowered [patent literature 1 (WO2005/100410). Thus, introduction of hydrogen into a polymerization reactor is an extremely effective method for the control of a molecular weight of the resulting olefin polymer. However, it is apparent that unlimited introduction of hydrogen for the purpose of controlling a molecular weight of an olefin polymer is not permitted. That is to say, in the case where polymerization is carried out under the conditions of a certain total pressure in a polymerizer and a certain temperature, rise of hydrogen partial pressure due to introduction of hydrogen causes lowering of a partial pressure of an olefin that is a polymerization monomer, and there occurs a problem of reduction in polymerization velocity particularly in the region of high hydrogen partial pressure. A polymerization reactor is restricted in its permissible internal total pressure because of design, and therefore, if excessive introduction of hydrogen is needed in, particularly, the production of an olefin polymer having a low molecular weight, the olefin partial pressure is extremely lowered, so that polymerization activity is sometimes lowered. On that account, desired is a polymerization catalyst capable of sufficiently lowering a molecular weight of the resulting olefin polymer by introducing a small amount of hydrogen and capable of controlling the molecular weight to a desired value, that is, a polymerization catalyst exhibiting a high responsiveness to hydrogen.
As described above, a molecular weight of the resulting olefin polymer decreases with a rise in polymerization temperature, and therefore, it is theoretically possible to control the molecular weight of the olefin polymer to a desired value by changing the polymerization temperature. However, for the reasons described below, control of a molecular weight of an olefin polymer by the polymerization temperature involves difficulties. First of all, in the control of a molecular weight of an olefin polymer to a desired value, the polymerization temperature cannot be raised up to a sufficiently high temperature in some cases because of withstand heat limit and withstand pressure limit based on design of a polymerizer itself or restriction due to heat stability of the resulting olefin polymer. On the other hand, the polymerization temperature is not decreased down to a sufficiently low temperature in some cases because the polymerization activity is lowered, or because in solution polymerization or the like, concentration of an olefin polymer cannot be raised due to increase in viscosity of a polymerization solution, and the productivity is lowered. Moreover, in the case where olefin polymers of many kinds different in molecular weight are continuously produced by one polymerization equipment, said case being popular to a person skilled in the art, a long time is frequently needed in order to stabilize the temperature of the polymerization solution to a desired value after changing the temperature. During this long time, lowering of productivity is brought about. Such an influence becomes conspicuous as the size of the polymerization equipment is increased. Accordingly, when a molecular weight of the resulting olefin polymer is controlled to a desired value in the industrial production of the olefin polymer, changing the amount of hydrogen added while maintaining the polymerization temperature at a certain value is preferably used by a person skilled in the art rather than changing the polymerization temperature.
Therefore, there has been eagerly desired a catalyst simultaneously achieving production of an olefin polymer having a high molecular weight in order to keep the polymerization temperature high and such a high responsiveness to hydrogen that an olefin polymer having a desired molecular weight is obtained by adding a small amount of hydrogen without lowering activity.